The present invention relates generally to optical fibers, and more particularly to measuring the birefringence autocorrelation length in optical fibers.
Fiber optic technology and fiber optic transmission systems have revolutionized telecommunications. The main driving force behind this revolution is the promise of extremely high communications bandwidth. A single wavelength of modulated laser light can carry vast amounts of information sufficient to comprise literally hundreds of thousands of phone calls or hundreds of video channels.
In a fiber optic transmission system, information may be conveyed on multiple optical signals, each centered on a different wavelength. Digital signals can be represented by modulating laser light, e.g., by rapidly turning the laser light on and off to represent the two possible states of a digital bit, i.e., “1” and “0”, or “on” and “off”. This process may be referred to as on-off-keyed modulation. Other types of modulation of the light also exist to convey information, including both digital and analog forms. The wavelength signal is generally emitted from a device called an optical transmitter such as a laser. In the frequency domain, this signal is centered about a frequency of the optical carrier, such as, for example, 193,000 GHz, but the signal may also include numerous frequency components spaced very closely about the nominal center frequency.
An optical signal is transmitted in a fiber optic transmission system using, generally, an optical transmitter, which includes a light source or laser, an optical fiber, optionally optical amplifier(s), and an optical receiver. A modulated optical signal arriving at an optical receiver must be of sufficient quality to allow the receiver to clearly distinguish the modulation pattern of the light sent by the transmitter (e.g., the on- and off-pulses). Noise, attenuation, and dispersion are a few of the impairments that can distort an optical signal and render the optical signal marginal or unusable at the receiver. The distortion of an optical signal makes it extremely difficult or impossible for an optical receiver to accurately detect and reconstitute the original signal. Some of these distortions broaden the various light pulses, potentially resulting in overlapping pulses so that it is difficult to distinguish the various pulses from each other. This may result in increased errors in transmitting information. Conventionally, a properly designed optical link can maintain a sufficiently low Bit Error Rate (“BER”).
Dispersion can be a major contributor to the distortion of an optical signal, which may increase the BER of the optical signal. The distortion caused by dispersion generally increases as the bandwidth of each light pulse or data rate increases and as the optical fiber transmission distance increases.
One type of dispersion is Polarization Mode Dispersion (“PMD”) which is an effect related to the polarization of the optical signal. It is well known that monochromatic light (as from a laser) is polarized and that, for a given beam of light, the state of this polarization may be expressed with excellent approximation in terms of two principal polarization modes represented by two orthogonal axes that are normal to the axis of propagation. Thus, an optical signal may be considered as a superposition of two polarization signals, each aligned with the two principal polarization modes of the fiber. As such an optical signal propagates through the optical fiber, the two polarization signals travel or propagate at different speeds, due to the birefringence (or double refraction) of that fiber. The different velocities associated with the two different principal polarization modes result in one of the two polarization signals leading the other polarization signal. The delay between the leading signal and the trailing signal is referred to as the Differential Group Delay (“DGD”) (denoted by τ). This speed difference in the two polarization signals causes pulse broadening and restricts the usable bandwidth of each optical carrier.
At each frequency, two polarization axes may be identified in a fiber—a fast axis and a slow axis. Light that is polarized along the fast axis has a greater velocity than light which is polarized along the slow axis. Light polarized along either of these axes is said to be in a principal polarization state. Any signal propagating in that fiber can be expressed as a combination of signals polarized along these two axes—i.e. as a combination of the two principal polarization states. The “polarization state” of the light expresses how much of the light is polarized along each of the axes as well as the relative phase between these two components. The light in a pulse is usually a mixture of both polarizations and will therefore spread due to the velocity difference between the portion of the light polarized along the fast and slow axes. This velocity difference is due to the birefringence of the fiber, a characteristic or property of the material. And just as the state of polarization can be thought of as having a specific orientation, so too the local birefringence has an orientation with respect to the fiber.
In many optical fibers, not only is birefringence present, but the birefringence is nonuniform and varies along the length of the fiber. In other words, the local birefringence, β(z), as a function of the position z along the fiber, may vary in either magnitude or orientation.
Many different phenomena may contribute to causing the birefringence, including, for example, asymmetrical fiber optic transmission media, mechanical stresses and strains applied to the fiber optic media, and other physical phenomena such as temperature gradients and changes. With these multiple phenomena affecting the birefringence in an essentially random fashion along a fiber, β becomes a statistical quantity. Depending on the number and magnitude of these sundry causes, β(z) may vary slowly or rapidly along the fiber. Beginning at an arbitrary point z=0 in the fiber, typically there is a small distance over which P(z) changes very little, and then there will be a distance over which it is changing, becoming less correlated with the magnitude and orientation of β(0). Following that, β(z) will become completely decorrelated with β(0). Thus, a calculation of the normalized autocorrelation of P for a given fiber will have a specific width (e.g., when the autocorrelation falls from 1 to ½) which corresponds to the distance along the fiber for which the local birefringence changes only slightly in orientation or magnitude. This quantity is commonly referred to as the birefringence autocorrelation length.
A fiber traditionally has to be cut in order to measure the local birefringence β(z) or its autocorrelation length. This destructive procedure is necessary because the birefringence varies along the fiber's length and spatially-resolved polarization-sensitive optical techniques (such as polarization optical time-domain reflectometery, P-OTDR) cannot determine all types of birefringence. Specifically, P-OTDR is incapable of sensing any local circular birefringence in the fiber because the probe light must retrace its path through the exact same local birefringence, but in the reverse direction.
In order to determine the birefringence autocorrelation length in a fiber, the fiber is cut into particular sections, and the DGD, for example, is determined for each fiber section. The fiber's DGD is then plotted against the fiber's length to determine the relationship between the fiber's DGD and its length. It has been found that when local birefringence is constant in orientation and magnitude, the DGD associated with the fiber is a linear function of the fiber's length. When β varies significantly over length, the relationship between a fiber's DGD and its length is proportional to √{square root over (L)}. The crossover point between a linear relationship of DGD with length and a √{square root over (L)} relationship is often viewed as representing the birefringence autocorrelation length.
The birefringence autocorrelation length is often viewed as being extremely important in describing the polarization properties of a fiber and so there is great interest in measuring a fiber's birefringence autocorrelation length. However, as discussed, the use of destructive techniques to measure this value introduces a new set of problems. For example, connecting the many segments of a fiber after cutting it to determine the birefringence autocorrelation length may introduce abnormalities and deformities (such as additional loss and reflections). But, even with optically perfect splices, such a procedure is very labor intensive and therefore costly.
As a result, there remains a need to determine the birefringence autocorrelation length of a fiber in a non-destructive manner.